The present invention relates to a magnetic recording medium for use in a magnetic disk apparatus and the like. More specifically, the present invention relates to a magnetic recording medium having excellent properties when it is used with a head employing a reproducing device making use of a magnetoresistive effect, represented by an inductive-MR composite head.
A magnetic disk apparatus or the like has been recently designed to have a higher recording density. To keep up with this tendency, improvement in the signal-to-noise ratio (hereinafter referred to as xe2x80x9cSNRxe2x80x9d) is required and further the isolated waveform half width (hereinafter referred to as xe2x80x9cPW50xe2x80x9d) needs to be reduced.
In order to attain higher SNR and narrower PW50, increase of coercivity is effective, and for increasing coercivity, use of a material having a higher Ku for the magnetic recording layer is effective. The technique of attaining higher coercivity due to the increase of Ku can also be an effective means for solving the problem of thermal decay standing out in recent years, that is, a problem such that signals once recorded decrease with the passage of time.
If the coercivity (Hc) is elevated so as to improve SNR and PW50, however, problems may arise, for example, the signals once recorded are not deleted even if those signals are overwritten (deterioration in overwrite (hereinafter referred to as xe2x80x9cOWxe2x80x9d) property) or signals cannot be written in the track edge area (deterioration in off-track property).
For solving these problems, an inductive-MR composite head obtained by compounding an inductive head optimized for writing and a head employing a reproducing device making use of a magnetoresistive effect and thereby having improved reading sensitivity is used.
On the other hand, a technique of providing a soft magnetic layer, so-called keeper layer, on a magnetic recording medium has been recently used with an attempt to improve the electromagnetic property or thermal decay. Examples of magnetic recording media having a soft magnetic layer include those disclosed in B. Gooch, R. Niedermeyer, R. Wood and R. Pisharody, IEEE Transactions on Magnetics, Vol. 27, No. 6, p. 4549 (1991), JP-A-7-169037 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-A-10-116412.
The magnetic recording medium disclosed in Gooch et al. is disadvantageous in that unless a bias flux for saturating the soft magnetic layer is added from the writing core of the inductive head when the recorded data is reproduced, a leakage flux is not induced from the medium surface and good reproducing signals cannot be obtained.
For this reason, the magnetic recording medium disclosed in Gooch et al. cannot be used with an inductive-MR composite head described above which is widely used at present.
The magnetic recording medium disclosed in JP-A-7-169037 is designed to cope with a reading-writing head of a bias current 0 (for example, inductive head) and difficulties in attaining high recording density and reduction in the noise arise.
Under the current circumstances requiring higher recording density, a method of using an inductive head or a method incapable of using a reading device making use of magnetoresistive effect, such as MR or GMR head, in the reproduction is not practical any more.
More specifically, use of an inductive head or the like approaches a limit in view of the reduction of noise and the increase of recording density and the limit cannot be overcome. However, when a head making use of magnetoresistive effect as represented by MR and GMR is used, the isolated waveform output is as high as, for example, from 3 to 10 times that of the inductive head, so that the magnetic recording film can be made thin and thereby low noise and high recording density can be achieved.
The magnetic recording medium disclosed in JP-A-10-116412 is stated to be usable with a commercially available inductive-MR composite head. However, in the case of high-coercivity magnetic recording medium for high-density recording now in wide use where a soft magnetic layer having a permeability as low as about 100 to 1,000 is used, the recording properties cannot be improved and electromagnetic properties cannot be satisfied either.
In the case of conventional magnetic recording media without a no soft magnetic layer, Ni and the like in the NiP layer on the surface of a non-magnetic substrate diffuses from the surface of the magnetic recording medium through defects such as scratches or pits and results in corrosion in some cases.
The present invention has been made under these circumstances and an object of the present invention is to provide a magnetic recording medium capable of increasing SNR and PW50 without causing any deterioration in the OW property and the off-track property and moreover, favored with a sufficiently high corrosion resistance.
The above-described object is attained by a magnetic recording medium comprising a non-magnetic substrate having thereon a non-magnetic underlayer, a magnetic recording layer, a soft magnetic layer and a protective layer, wherein the coercivity is 2,500 Oe or more and the thickness of the soft magnetic layer is from 5 to 50 xc3x85.